1. Field of the Invention
The present invention relates to a portable timepiece having a structure for preventing an accidental rotation of a crown of the portable timepiece.
2. Description of the Prior Art
A conventional portable timepiece such as a diver's portable timepiece has a structure for locking a crown of the portable timepiece by means of screw-engagement, which is referred to as “screw-locking” or “screw-locking mechanism” in this specification, so as to prevent an accidental rotation of the crown while the portable timepiece is being carried with a user.
In the screw-locking structure, a winding stem pipe is attached to a case band containing a portable timepiece movement. A male screw is formed around the outer periphery of a pipe end of the winding stem pipe which pipe end projects outwardly from the case band. A female screw of a crown head to be fitted to the pipe end is brought into screw-engagement with the male screw. In a normal condition, the crown is allowed to be screw-locked by bringing the crown head into screw-engagement with the outer periphery of the pipe end of the winding stem pipe. For controlling the portable timepiece movement, the screw-engaged crown head is released from the male screw and the crown is pulled before a winding stem housed within the winding stem pipe is rotated.
In the conventional portable timepiece having this screw-locking structure, the attachment portion of the winding stem pipe is generally waterproofed by brazing the winding stem pipe to the case band (see JP-A-57-46181 (from page 1, right column line 8 to page 2, left column line 7, FIGS. 1 and 2), for example).
In another known portable timepiece having an additional male screw on a winding stem pipe apart from a male screw with which a crown screw-engages, the additional male screw is screwed into a pipe attachment opening of a case band, and along with the screwing action a waterproof packing is inserted between the case band and a crown head so as to waterproof the winding stem pipe attachment portion.
In a portable timepiece having a screw-locking mechanism, the screwlocking effect may be lowered by abrasion or wear between a female screw of a crown and a male screw of a winding stem pipe under the condition of a screw-engagement with each other as a result of impact caused by dropping or banging the timepiece, repetitive rotations of the crown or for other reasons.
When this situation occurs, the crown and its surroundings of the portable timepiece disclosed in JP-A-57-46181 cannot be replaced because the winding stem pipe which needs to be removed from the band case before the replacement is brazed to the case band. Thus, the entire portable timepiece casing assembly including the case band has to be replaced.
For a portable timepiece in which a winding stem pipe is screwed into a case band, theoretically the crown and its surroundings including a winding stem pipe are considered to be replaceable. In this structure, however, a rotational force is exerted on the winding stem pipe every time a crown is brought into screw-engagement with the winding stem pipe and released therefrom. This rotational force loosens the winding stem pipe screwed into the case band, resulting in possible lowering of waterproofing effect offered by a waterproof packing.
As a solution for this problem, such a method is sometimes adopted in which the screw-engagement portion between the winding stem pipe and the case band is filled with an adhesive. According to this adhesive method, it is in practice often difficult to remove the winding stem pipe from the case band since the applied amount of the adhesive is hard to be controlled. As a result, the crown and the surrounding components cannot be replaced and thus the entire portable timepiece casing assembly including the case band must be replaced when the screw-locking effect is lowered.
An object to be achieved by the present invention is to provide a portable timepiece whose crown and surrounding components are replaceable when a screw-locking function is lowered.